mmc: sdhci: rename platform_8bit_width to platform_bus_width

The 8bit in the function name is misleading. When set, it will be
used to set the bus width, regardless of whether 8bit or another
bus width is requested, so change the function name to
platform_bus_width.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Chris Ball <cjb@laptop.org>